1. Field of the Invention
The present invention relates to a method of fabricating an optical aperture. Particularly, the invention relates to a method of forming an optical aperture at a probe used in a near field light device for irradiating and detecting near field light.
2. Background Information
There is used a scanning probe microscope (SPM) represented by a scanning tunnel microscope (STM) or an atomic force microscope (AFM) for observing a very small area of a nanometer order at a surface of a sample. Although SPM can acquire an image of a resolution dependent on a shape of a front end of a probe by scanning a probe having a sharpened front end at a surface of a sample via interactive operation produced between the probe and the surface of the sample such as tunnel current or atomic force as an object of observation, a restriction imposed on the sample to be observed is comparatively severe.
Hence, currently, attention is attracted to a near field optical microscope (SNOM) capable of observing a very small area of a surface of a sample by constituting an object of observation by interactive operation caused between near field light and a probe produced at the surface of the sample.
According to the near field optical microscope, near field light is irradiated to the surface of the sample from an aperture provided at a sharpened front end of an optical fiber. The aperture is provided with a size equal to or smaller than a diffraction limit of a wavelength of light introduced into the optical fiber, which is, for example, a diameter of about 100 nm. A distance between the aperture formed at the front end of the probe and the sample is controlled by the technology of SPM and its value is equal to or smaller than the size of the aperture. In this case, a spot diameter of near field light on the sample is substantially the same as the size of the aperture. Therefore, by scanning near field light irradiated to the surface of the sample, observation of optical properties of the sample at the very small area can be carried out.
In addition to use as a microscope, there is enabled an application as high density optical storage recording in which by introducing light having a comparatively large density toward a sample via an optical fiber probe, near field light having a high energy density is produced at an aperture of the optical fiber probe and a structure or a physical property of the surface of the sample is locally changed by the near field light. There has been carried out a trial of enlarging a front end angle of the front end of the probe to acquire near field light having a large intensity.
According to the devices utilizing near field light, formation of the aperture is most important. As one of methods of fabricating the aperture, there is known a method disclosed in Japanese Patent No. 21201-1993. In this method, an optical shielding film is deposited on a sharpened light waveguide. According to the method of fabricating the aperture, by pressing the sharpened light waveguide having the optical shielding film against a hard flat plate by a very small amount and in a controlled manner using a piezoelectric actuator, the optical shielding film at the front end is subjected to plastic deformation.
Further, as a method of forming an aperture, there is a method disclosed in Japanese Patent Laid-Open No. 265520/1999. According to the method of fabricating the aperture, an object of forming the aperture is a front end of a projection formed on a flat plate by focused ion beam (FIB). The method of forming the aperture is carried out by irradiating FIB from a side face to an optical shielding film at the front end of the projection to thereby remove the optical shielding film at the front end of the projection.
However, according to the method of Japanese Patent No. 21201/1993, the aperture can be formed respectively for only a single piece of light waveguide. Further, according to the method of Japanese Patent No. 21201/1993, the amount of pressing must be controlled by the piezoelectric actuator having several nm of movement resolution and accordingly, the aperture forming apparatus must be placed under an environment having insignificant influence by other apparatus or vibration of air. Further, the procedure for bringing a light carrier rod into orthogonal contact with the flat plate is time consuming. Further, a mechanical translation base having a large moving amount is needed other than the piezoelectric actuator having a small moving amount. Further, a control apparatus is needed to control the pressing amount by the piezoelectric actuator having the small movement resolution, and a time period of several minutes is required to control the amount of pressing to thereby form the aperture. Therefore, there are needed large-scaled apparatus such as a high voltage power source and a feedback circuit for fabricating the aperture. As a result of the foregoing, the cost for forming the aperture is high.
Further, according to the method of the Japanese Patent Laid-Open No. 265520/1999, although the object of fabrication is the projection on the flat plate, since the aperture is formed by using FIB, a time period required for forming a single one of the aperture is as long as about 10 minutes. Further, the sample must be placed in vacuum in order to use FIB. Therefore, there poses a problem that fabrication cost required for forming the aperture is increased.